1. Field of the Invention
This invention relates to generic Digital Subscriber Line systems and voice communications systems and more particularly to methods and systems for combining symmetric Digital Subscriber Line signals and analog voice signals.
2. Description of Related Art
With the increased usage of Internet services and the desire or need for other high data rate signals, consumers require equipment that can handle data signals at a rate greater than that available via traditional analog voice modems, which currently have a maximum data rate of 56K bits per second (“BPS”). Recently, business and residential consumers are using Digital Subscriber Line (“DSL”) modems as part of a system for getting high speed access to the Internet. DSL type of service is currently being offered by many telecommunication access service providers, which include numerous independent access providers and traditional telephone companies. There are many variations of DSL including High-Bit-Rate Digital Subscriber Line (“HDSL”), Asymmetric DSL (“ADSL”), and Symmetric DSL (“SDSL”). (“Asymmetric” refers to the fact that the upload and download bit rates are not equal and “Symmetric” refers to the fact that the upload and download bit rates are the same.)
HDSL and SDSL are both symmetrical DSL signals that use the same modulation technique. HDSL systems use two pairs of twisted wire, operate at a fixed data rate (equivalent rate to T1 in North America and E1 outside North America) and are mainly used as a replacement for T1 and E1 network connections. ADSL and SDSL systems use a single twisted pair of wires and are capable of operating at variable data rates. A description of an exemplars DSL system employing 2B1Q modulation is described in Bellcore TA-NWT-001210 Issue 1, October 1991. Generic Requirements for High-Bit-Rate Digital Subscriber Lines which is hereby incorporated by reference for its teachings on DSL systems and 2B1Q modulation.
These DSL services are generically called xDSL. xDSL service reaches its customers by using copper wires that extend from a telephone company's Central Office (“CO”) to a residence or business. Currently, telephone companies mostly offer ADSL service, which is a service targeted at their consumer or residential customer base and offer businesses T1 service using HDSL. Independent or alternative access providers are also in the access market and offer mostly SDSL. There are several types of SDSL and HDSL modems available. The most common type of modulation format the modems employ is a 2B1Q (two bits per quadrant or symbol) format, which is a simple four level signal modulation format known as Pulse Amplitude Modulation (“PAM”). For the sake of clarity, during the remainder of the document, only SDSL will be described although the invention applies equally to both SDSL systems and single wire pair HDSL systems that use 2B1Q modulation.
FIG. 1 (Prior Art) is a diagram of a prior art communication system 270 in which both voice and data (via SDSL access) are delivered through independent means and equipment to an individual customer. Voice and SDSL is delivered in this way because of the inability of voice and such data signals to coexist on the same connection. As shown in this figure, Customer premise equipment (“CPE”) 240 and Central Office (“CO”) equipment 250 receive (a) traditional voice communication via a twisted pair of wires 173 and (b)SDSL communication via a separate twisted pair of wires 214 commonly dedicated to transferring only SDSL signals. The CPE 240 includes call signal destination equipment 144, an SDSL modem 212, and digital signal destination equipment 148. The CO equipment 250 includes a telephone switch 174 and an SDSL Modem and DSL Access Multiplexer (“DSLAM”) 213. As shown in FIG. 1, the SDSL Modem/DSLAM 213 is coupled to the Internet 180 by a Router/Internet Service Provider (“ISP”) 260, the connection thereto is known to one of skill in the art.
In this FIGURE, the central office (“CO”) equipment 250 of a telephone company provides voice communication on the twisted pair of wires 173 between the call signal destination equipment 144 in the CPE 240 and a public switch telephone network (“PSTN”) 190. The call signal destination equipment 144 may be any voice signal processing apparatus or telephone system such as a telephone keyset, public branch exchange (“PBX”), or voice band modem or fax. It is noted that the PSTN 190 may be any telephone system including PSTN, PBX, or plain old telephone system (“POTS”). As noted above, the CO equipment 250 includes a telephone switch 174 that transfers or switches analog voice signals to the twisted pair of wires 173 when the signal is addressed to the CPE 240 and to a connection into the public switched telephone network (“PSTN”) 190. Voice and signaling signals appear on wires 173 and consist of voice band signals whose frequency ranges from zero to 3.4 kHz, a batter feed voltage (DC at −48 volts) and an AC ringing high voltage (20 Hz at 90 volts RMS). Transient signals may also appear on the twisted pair of wires 173 during changes in line conditions such as at the beginning and end of a ringing signal, the taking of a phone off-hook during ringing and not during ringing, the placement of a phone on-hook, the switching of ringing to battery feed signal and the switching of the batter feed signal to the ringing signal.
As also shown in this FIGURE an SDSL access provider (either the telephone company or an alternative service provider) uses additional equipment, an SDSL Modem//DSLAM 213 inside the CO 260 for providing data communication on the twisted pair of wires 214 between the CPE 240 and the Internet 180 via a Router/ISP 260. The SDSL Modem/DSLAM 213 includes a digital subscriber line access multiplexer (“DSLAM”) with SDSL modems.
The SDSL (2B1Q modulation format) modem inside the SDSL Modem/DSLAM 213 converts SDSL signals (which are modulated analog data signals) on the twisted pair of wires 214 to digital signals. The SDSL Modem/DSLAM 213 also multiplexes many SDSL modem signals into a single high-speed signal and passes that signal on to the ISP/Router 260. The ISP/router 260 interconnects the SDSL Modem/DSLAM 213 to the Internet 180.
Similarly, when a digital data signal is transmitted from the Internet 180 to the CPE 240 via line 214, the ISP/router 260 transfers the Internet traffic to the SDSL Modern/DSLAM 213. Any digital data that is addressed to the CPE 240 is then sent to the appropriate SDSL modem of the SDSL Modem/DSLAM 213. The SDSL modem in the SDSL Modern/DSLAM 213 converts this digital data to an SDSL signal where the SDSL signal is modulated analog signal. The SDSL signal is transmitted to the CPE 240 Via the twisted pair of wires 214. The SDSL (2B1Q) signals have a frequency range starting from DC (0 Hz) to an upper frequency that is one half of the assigned data rate. For example: when the data rate of the service being offered is 384 kbps, the frequencies may range from 0 Hz to 192 kHz.
As noted above the CPE 240 includes an SDSL modem 212 and digital signal destination equipment 148. The SDSL modem 212 demodulates SDSL signals (as noted, the SDSL signals are analog data signals) received on the twisted pair of wires 214 and provides a digital signal that is transferred to the digital signal destination equipment 148 via line 145. Line 145 may be an Ethernet cable or serial cable capable of transferring a digital signal. The digital signal destination equipment 148 may be networking equipment such as a hub, router or switch or an individual computer. The SDSL modem 212 also modulates digital signals received from the digital signal destination equipment 148, using a modulation or encoding technique compatible with that used by the SDSL modem of SDSL Modem/DSLAM 213, and transmits them to the SDSL Modem/DSLAM 213 on the twisted pair of wires 214.
The twisted pairs of wires 173 and 214 are typically owned by the local telephone company and stretch between the CO and a demarcation point just inside a building, which is known as the Minimum Point of Entry (“MPOE”). The wiring (173 and 214) that connects between the MPOE and the CPE 240 is privately owned. In an office buildings that contains more than one tenant, the MPOE and the privately owned wiring is housed in a single wiring closet that is common for all tenants. Thus, when the CPE 240 is located in an office of a large building, the common wiring closet of the building may be located several hundreds of feet from the call signal destination equipment 144 and the SDSL modem 212. In many cases when the line 214 is being installed from the CO equipment 250, a consumer will not have the second twisted pair of wires 214 from the MPOE (or from the common wiring closet) to the CPE 240 and so will be required to install them.
In older buildings, adding additional sets of wires for each digital signal destination may be prohibitively costly and also time consuming, delaying the implementation of such high speed data access. SDSL communication systems, however, were not designed to operate with the telephone voice systems. SDSL signals and voice signals are incompatible and thus must use the separate wires 173 and 214 to transfer SDSL and voice signals from a CO to a building. One element of their incompatibility is that SDSL signals and voice signals overlap or interfere with each other (voice: 0 to 3.4 kHz and SDSL: 0 to hundreds of kHz). SDSL modems can not tolerate the voice signal mixed with its modem signal and humans can hear extremely low audio signal levels and thus also can not tolerate the SDSL signal. Another element is that SDSL modems do not contain algorithms for handling transient line conditions such as those outlined above. SDSL signals are modulated using a very simple modulation method and thus intolerant of interference and SDSL modems do not have any external filtering.
In contrast, ADSL communication systems and equipment were designed to share one set of twisted copper pair of wires from the CO to a building with an analog voice signal from a telephone company. The system characteristics of ADSL communication that allows it to coexist with voice signals include: 1) a large non-overlap or separation of signals between the maximum voice frequency (0 to 3.4 kHz) and the lowest ADSL frequency (22 kHz and above): 2) strong analog ADSL separation filters: 3) a complex ADSL modulation scheme: and 4) Digital Signal Processing (“DSP”) algorithms that operate inside the ADSL modem to handle transient line conditions.
ADSL service has several disadvantages, however, for business use over SDSL service. In particular, ADSL signals are non-symmetrical: in other words the download signal rate is greater than the upload signal rate (from the CPE to ADSL service provider). The asymmetry is a disadvantage for commercial use because business customers typically need to move large blocks of data both to and from their establishment.
Consumers desiring the many advantages of SDSL service ideally would like to make use of their existing voice-signal wiring rather than having to install an additional line (part of line 214 of FIG. 1) between their CPE and a remote location in the same building (such as the MPOE or a wiring closet). Consequently, the need exists for a system and method that can combine SDSL signals with analog voice signals on a single twisted pair of wires without corrupting either signal even in the presence of voice and voice signaling signals.